Magic Springs and Seatbouncing are back!

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11/10/2012 8:45 AM

Haven't been here for a long time. I'll throw this out there just to bend your brains a little.

Just saw this slow-mo video and thought of an age old debate about magic shock springs and seat bouncing.

TFS & Nerd had it all figured. I of course had a complete lack of understanding of Newtonian Physics and Dynamics after a failed education and degree in Mechanical Engineering. (That is to be read rhetorically.)

For the life of me, given the infinite wisdom of those two, I can't seem to use their explanations of rebound and center of gravity and maximization of ramp projectile magical physics to explain how mookies bike bounces off a flat surface. (Again, read rhetorically.)

TFS gave us a sound mental experiment. Strap a bike to the floor fully compressed and then cut the straps and the bike will rebound zero inches off the ground because the dampening of the suspension. Therefore seatbouncing is a mysterious physical phenomena which alludes us. We feel like it comes from the springs and the rebound, but this just isn't the case according to the All knowing, All powerful, Infinite wisdom of the resident photographers.

Is there a problem with Mookies suspension setup? Is it not properly damped? How does that rear wheel come off the ground before the rear suspension has fully rebounded at 2:15?

Magic Springs?

Is it possible for a rider to use this magic moment caught on camera? Instead of the bounce on the landing, use it on take off? Could he load the rear suspension on the face of a jump, and then just prior to the launch of the jump, unload the rear suspension and get a little extra BOOST? A seat bounce?

Is it possible to have a bike take off from the ramp before the rear suspension has completely rebounded? Thus LIFTING OFF the ramp rather than merely rolling off the end of the ramp?

I have no idea if this will connect with anyone. It was a fun debate years ago and led to a very fun exciting application of high school physics with a complicated twist of fluid dynamics.

Enjoy!

http://www.racerxonline.com/2012/11/09/malcolm-stewart-at-1000-fps

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11/10/2012 8:46 AM
Edited Date/Time: 11/10/2012 8:47 AM

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11/10/2012 8:50 AM

"TFS gave us a sound mental experiment. Strap a bike to the floor fully compressed and then cut the straps and the bike will rebound zero inches off the ground because the dampening of the suspension. Therefore seatbouncing is a mysterious physical phenomena which alludes us."

I wonder what would happen if the bike were to hit a 4" tall bump traveling 40mph when you cut those straps.

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11/10/2012 9:02 AM

TFS said...

/thread

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11/10/2012 9:12 AM

If you took the shock off all together & bolted a solid piece of steel in it's place, the wheel would still bounce off the ground at times. Therefore the shock spring has nothing to do with bouncing.

Duuhhh.


tongue

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HAF

11/10/2012 11:13 AM

Simply put, a seat bounce works primarily because of the resulting weight transfer affecting the rider, not the bike.

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11/10/2012 11:20 AM

TFS had a flawed example. Put the bike on a scale and see if it gets lighter.

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11/10/2012 2:59 PM

I have no idea what the heck you are talking about.... What seat bounce?

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Never try to argue with idiots; they will only bring you down to their level.....and being more experienced, they will beat you at their own game!

2020.5 KTM 450 SXF FE
2006 KX250

11/10/2012 3:12 PM

Seat bouncing works on a BMX bike. No spring on a BMX bike.

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11/10/2012 4:19 PM

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"Life is Too Short To Last Long"

11/10/2012 5:19 PM

Photo
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-OC
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11/10/2012 6:22 PM

Basic kinematics, motodave15.

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11/10/2012 6:27 PM
Edited Date/Time: 11/10/2012 6:30 PM

Kinetic energy conbined with forward motion and skill...on the second one, I do believe that the track builder will provide a bit of a lip to test the testies of the ridersmile

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Germany 1975 250 CZ Centerport, laydown shocks, mikuni with reed valve, Marzocchi forks with me as the motopilot

11/10/2012 6:32 PM

bikes do bounce ya know

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KEEP WORKING, MILLIONS ON WELFARE DEPEND ON YOU!

11/10/2012 6:43 PM

reded wrote:

"TFS gave us a sound mental experiment. Strap a bike to the floor fully compressed and then cut the straps and the bike will rebound zero inches off the ground because the dampening of the suspension. Therefore seatbouncing is a mysterious physical phenomena which alludes us."

I wonder what would happen if the bike were to hit a 4" tall bump traveling 40mph when you cut those straps.

The skill aspect of bouncing a bike over an obstacle is……that the rider as they squeeze the bike actually assist in lifting the bike…the speed and the application of the throttle at the correct moment further assist the effort

JMHO

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Germany 1975 250 CZ Centerport, laydown shocks, mikuni with reed valve, Marzocchi forks with me as the motopilot

11/10/2012 6:48 PM

I'm sure many of you guys are familiar with seat-bouncing when it goes wrong -- I've ended up on my head multiple times. Because of the fact that when done wrong, seat-bouncing sends the bike into a nose dive, my feeling is that the spring provides some lift.

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11/10/2012 7:01 PM

One of David Bailey's videos showed him, on totally flat ground, getting the bike off the ground to clear a ditch. The physics involved in this seat bounce skill is incredible. But it works. Any decent rider knows this.

And a big thumbs down on the endo scenario when it goes wrong lol.

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Ironman Texas, May 17th, 2014, I became an IRONMAN.

11/10/2012 8:39 PM

It's the tire! It acts as an undampened spring! Like a basketball. silly

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11/10/2012 8:48 PM
Edited Date/Time: 11/10/2012 9:27 PM

The bike is dampened through the spring but the rider is not, that's what is missing here. As the bike rebounds, it pushes the rider up, the bike too but the bike has damping which obviously "damps" the bike from "springing" up. However, the rider is not "dampened" so he keeps moving up and if he does it right will pull the bike with him getting extra lift, if not, could end up in endo. So you ask, why can't you "peg" bounce? because the pegs are closer to the axis of rotation and therefore there is not nearly as much push from the spring at the pegs as there is on the seat (further from axis of rotation). Has to do with angular velocity (physics term) which is the same for both but the actual linear velocity is greater at the seat which we take advantage of when we seat bounce...nuff said.

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11/11/2012 6:19 AM

You CAN "peg bounce" while standing, you push the bike into the face of the jump with both your legs and your hands...compresses the suspension even more than the g-forces, and you will jump higher/farther...

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Ironman Texas, May 17th, 2014, I became an IRONMAN.

11/11/2012 6:47 AM

So look who showed up solely to start a fight

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11/11/2012 6:59 AM

flarider wrote:

So look who showed up solely to start a fight

Who's fighting?

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Ironman Texas, May 17th, 2014, I became an IRONMAN.

11/11/2012 9:53 AM

Same as bunny hopping a bmx, its just a transfer of weight in the right order.



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11/11/2012 11:22 AM

You would have loved this old forum back in the day. www.seatbounce.com/

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11/11/2012 5:21 PM

I second what adam445 said. The seat bounces comes from the motion of the riders body. On a bike or skate or anything with wheels you can exert this same force. On a ramp, with a bike, skate, roller blades, or anything with wheels, you can "pump" to make yourself go faster. Before the ramp, you slowly compress your legs and once you hit the apex or the ramp you quickly straighten your legs which gives you more boost. I don't have a video to show you, but this is what all good bikers and skaters do. On a motorcycle, instead of compressing your legs like you on a bike or skate, you compress the bike's suspension and the upward motion from that rebounding, just legs extending your legs, pushes your body upwards which translates into upward motion. It's mot magic.

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i BMX

11/11/2012 5:30 PM

xray52 wrote:

The bike is dampened through the spring but the rider is not, that's what is missing here. As the bike rebounds, it pushes the rider up, the bike too but the bike has damping which obviously "damps" the bike from "springing" up. However, the rider is not "dampened" so he keeps moving up and if he does it right will pull the bike with him getting extra lift, if not, could end up in endo. So you ask, why can't you "peg" bounce? because the pegs are closer to the axis of rotation and therefore there is not nearly as much push from the spring at the pegs as there is on the seat (further from axis of rotation). Has to do with angular velocity (physics term) which is the same for both but the actual linear velocity is greater at the seat which we take advantage of when we seat bounce...nuff said.

That's a damn good explanation. It's not about the bike!

Could the great seatbounce controversey be solved? Next thing you know Bigfoot will hold a press conference.

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11/12/2012 6:32 AM

gspointer wrote:

TFS had a flawed example. Put the bike on a scale and see if it gets lighter.

Exactly. The bike gets light because of the spring. Combined with the upward momentum of the rider and a precisely timed blip of the throttle.

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I have the right to remain awesome.

11/12/2012 6:42 AM

If you look at trials mountain biking. They can static bunny hop with no forward momentum up to about 48 inches. Thats not just inertia from the movement of the body but a little pedal kick at the right time to generate a motion. Their body uses that motion with the force of their own to generate extra lift.


Now I can bunny hop my street bike with a bit of weight transfer and a quick blip/slip of the clutch. Be it only mere inches off the ground but the rebound, combined with the acceleration plus my body weight pulling up is a greater force than the 400lbs holding it to the earth (gravity).

Trials motorcyclists can do the same to an even bigger extent going 20ft across gaps to the back wheel off of a flat platform to a landing point equal if not greater in height. Comes down to timing, balls and body weight transfer.

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CR250 07
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11/12/2012 8:14 AM

thephoenix wrote:

One of David Bailey's videos showed him, on totally flat ground, getting the bike off the ground to clear a ditch. The physics involved in this seat bounce skill is incredible. But it works. Any decent rider knows this.

And a big thumbs down on the endo scenario when it goes wrong lol.

That's the funniest thing about this whole argument. Any decent A or B rider uses the technique on just about every track. But Vital can teach you a lot of things, like how this fairly basic and straight-forward ability actually doesn't exist and that every rider who claims he seat bounces a section is really lying to himself and everyone else.

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11/12/2012 10:15 AM

I think seat bouncing has a lot more to do with the mass of the rider and bike following a path than suspension. Under normal conditions going up the face of a jump suspension and your legs are compressing due to the direction change of the mass from the takeoff ramp angle. This means the bulk of the mass (you and bike) did not follow the true path of the ramp resulting in your path through the air not being what it optimally could be. When you seat bounce any dampening is removed by you sitting on the seat and the suspension being fully compressed. This make the bulk of mass (you and bike) follow the true path of the ramp resulting in a truer projectile path based on launch angle. Scrub is complete opposite where you try and get the bulk of mass to follow the launch angle as little as possible by laying the bike over as it progresses up the ramp. Does this make any sense?

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